Escherichia coli O157:H7, and other Shiga-toxin (Stx) produced E. coli (STEC) can cause hemorrhagic colitis and its life-threatening sequelae. the hemolytic uremic syndrome or thrombotic thrombocytopenic purpura. Human STEC disease is primarily transmitted by eating contaminated bovine food products. STEC strains, both virulent and non-virulent to humans, inhabit the gastrointestinal tracts of most health cattle. Interestingly, despite intensive investigations, an explanation as to why cattle carry STEC has not surfaced. A complete understanding of the relationship between STEC and its bovine reservoir will have a major impact on human health since it could provide a means of eliminating this key source of food contamination. Stx belongs to a large family of ribosome-inactivating proteins (RIPs) found in a variety of plants and some bacteria, which share enzymatic activity and molecular structural motifs. The plant RIPs are known to have potent antiviral activity this project tests the hypothesis that Stx has anti-viral properties in cattle. If so, this property of STEC could explain why STEC persists in these animals. We have strong preliminary data showing that Stx specifically inhibits the hallmark spontaneous proliferation of bovine leukemia virus (BLV)- The broad goal is to investigate the mechanism of viral inhibition by Stx. Specific aims include: (1) characterizing the anti-viral effects of Stx in vivo (lymphocyte cultures) and in vitro (BLV-infected and healthy cattle), (2) determining molecular structure/function relationships of Stx antiviral activity and (3) determining anti-BLV activity of gastrointestinal STEC in vivo. The multi-disciplinary team of co-investigators for this project and includes a microbiologist with expertise in STEC and immunology, a ruminant animal scientist, a ruminant microbiologist, a virologist, and a statistician.